Page 267 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.59

weathering out of doors. All of this can be improved if antidegradants, carbon black, and
waxes are used. Nevertheless, NR cannot compete with essentially saturated rubbers (con-
taining few, if any, main-chain double bonds) such as EPDM, ACM, IIR, CSM, and so on.
On the other hand, NR vulcanizates have very good low temperature flexibility. Here
also, it is only surpassed by BR, which has a somewhat lower glass transition temperature,
T .
g
NR vulcanizates exhibit relatively low (favorable) values of compression set at ambient
and somewhat higher temperatures. At lower temperatures, compression set is less favor-
able, possibly due to a tendency for the rubber to crystallize. At more elevated tempera-
tures, poor heat resistance and cross-link rearrangement can have a detrimental effect on
compression set.
NR vulcanizates have very poor resistance to swelling in gasoline, mineral oils, and
other nonpolar liquids, swelling as much as 1000 percent or more by volume. The resis-
tance of NR vulcanizates to swelling by polar liquids such as water, alcohol, and others is
very good.
Uses. NR is used mostly in the form of solid rubber and to a lesser extent as latex. Mi-
nor amounts of NR are used in adhesives, rubber solutions (cements), art gum, and other
products. A relatively small amount of NR is used in the production of hard rubber (a
high-temperature reaction product of NR with about 30 weight percent of sulfur). Before
the introduction of other rubbers or elastomers, all rubber products were produced from
NR. Because of improved specific properties of many synthetic rubbers, NR has been re-
placed in many applications. This is especially true where resistance to heat and weather-
ing, and oil and solvent resistance are required. There are also economic reasons for using
other rubbers. The price of natural rubber can be high when demand is greater than what
can be produced by the existing rubber trees. With the exceptions of tires and possibly a
few other applications, NR is no longer the preferred elastomer. NR is well suited for tires
because of its relatively low heat buildup, tearing resistance, low-temperature flexibility,
fatigue resistance, and building tack.
In addition to the above, NR is important in the production of thin-walled products
such as surgical gloves, balloons, condoms, and so forth. Here, latex dipping is the method
of shaping or forming. Because of its allergen content, the human-contact products are
now threatened.
Its low damping and high elasticity allow NR to be used for producing vehicle suspen-
sion elements and bumpers. An interesting suspension element is a building support to
“tune” the structure to resist earthquakes. NR has also been used in supports for bridges.
Early work in the 1950s on laminated rubber bearings for bridges, now used to accommo-
date bridge deck movements, gave rise to the development of bearings for the base isola-
tion of whole buildings against ground-borne vibrations (e.g., underground railway
systems). These, in turn, were further developed in the mid 1980s and 1990s for bearings
to protect buildings against earthquakes.
4.5.3.2 Synthetic Polyisoprene (IR). Synthetic polyisoprene is similar to natural rubber
in chemical structure and properties. Although it has lower green strength, lower hot tear,
and inferior aging characteristics than NR, synthetic polyisoprene exceeds the natural
types in consistency of product, processing, and purity. In addition, it has better mixing,
extrusion, molding, and calendering characteristics. (Processing methods for vulcanizable
rubbers are discussed in a later section.)
The successful synthesis of stereoregular polyisoprene (IR) fulfilled a goal sought by
polymer chemists for nearly a century. The polymer chains in the early synthetics con-
tained mixtures of all possible molecular configurations joined together in a random fash-
ion. Specifically, they lacked the very high cis-1,4 structure content of the natural rubber
backbone that gives it the ability to undergo strain-induced crystallization.

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